U.S. patent application number 15/328751 was filed with the patent office on 2017-07-27 for silica coated quantum dots with improved quantum efficiency.
This patent application is currently assigned to KONINKLIJKE PHILIPS N.V.. The applicant listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Patrick John Baesjou, Marcel Rene Bohmer, Roelof Koole, Joep Lodewijk Peters.
Application Number | 20170211756 15/328751 |
Document ID | / |
Family ID | 51224849 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170211756 |
Kind Code |
A1 |
Koole; Roelof ; et
al. |
July 27, 2017 |
SILICA COATED QUANTUM DOTS WITH IMPROVED QUANTUM EFFICIENCY
Abstract
The invention provides a method for the production of a
luminescent material, (10) based on coated quantum dots (100),
comprising: (i) providing luminescent quantum dots (100) in a
liquid medium (20) wherein the luminescent quantum dots (100) have
an outer layer (105) comprising first cations and first anions; and
(ii) providing in a coating process a coating (120) on the outer
layer (105) of the quantum dots (100) in the liquid medium (20),
wherein the coating (120) comprises a silica coating; wherein
during the coating process, or after the coating process, or during
and after the coating process, the liquid medium (20) comprises one
or more of a third element and a fourth element, wherein the first
cation and the third element belong to the same group of the
periodic system, and wherein the first anion and the fourth element
belong to the same group of the periodic system.
Inventors: |
Koole; Roelof; (Eindhoven,
NL) ; Baesjou; Patrick John; (Eindhoven, NL) ;
Bohmer; Marcel Rene; (Eindhoven, NL) ; Peters; Joep
Lodewijk; (Eindhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
Eindhoven |
|
NL |
|
|
Assignee: |
KONINKLIJKE PHILIPS N.V.
Eindhoven
NL
|
Family ID: |
51224849 |
Appl. No.: |
15/328751 |
Filed: |
July 24, 2015 |
PCT Filed: |
July 24, 2015 |
PCT NO: |
PCT/EP2015/067026 |
371 Date: |
January 24, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 33/502 20130101;
F21K 9/64 20160801; C09K 11/02 20130101; C09K 11/025 20130101; B82Y
30/00 20130101; F21V 9/30 20180201; Y10S 977/892 20130101; Y10S
977/774 20130101; C09K 11/565 20130101; B82Y 40/00 20130101; Y10S
977/824 20130101; Y10S 977/95 20130101; C09K 11/883 20130101 |
International
Class: |
F21K 9/64 20060101
F21K009/64; C09K 11/88 20060101 C09K011/88; F21V 9/16 20060101
F21V009/16; C09K 11/02 20060101 C09K011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2014 |
EP |
14178703.6 |
Claims
1. A method for the production of a luminescent material based on
coated quantum dots, the method comprising: (i) providing
luminescent quantum dots in a liquid medium wherein the luminescent
quantum dots have an outer layer comprising first element (M1)
comprising cations and second element (A2) comprising anions; and
(ii) providing in a coating process a coating on the outer layer of
the quantum dots in the liquid medium, wherein the coating
comprises a silica coating; wherein during the coating process, or
after the coating process, or during and after the coating process,
the liquid medium comprises one or more of a third element (M3)
comprising ion and a fourth element (A4) comprising ion, wherein
the first element (M1) and the third element (M3) belong to the
same group of the periodic system, and are selected from the group
of metal elements, wherein the second element (A2) and the fourth
element (A4) belong to the same group of the periodic system, and
are selected from the group of non-metal elements, and wherein M1
and M3 are each independently selected from the group consisting of
Zn and Cd, and wherein A2 and A4 are each independently selected
from the group consisting of S, Se and Te.
2. The method according to claim 1, wherein the one or more of the
third element (M3) and the fourth element (A4) are available in the
liquid medium in a concentration of at least of 10 mM.
3. The method according to claim 1, wherein one or more of the
following applies: M3=M1 and A4=A2.
4. The method according to claim 1, wherein the coating process is
executed in micelles containing said quantum dots.
5. The method according to claim 1, wherein during the coating
process the liquid medium comprises the third element (M3)
comprising ion and does substantially not comprise the fourth
element (A4) comprising ion, and wherein after the coating process
the liquid medium comprises the fourth element (A4) comprising
ion.
6. The method according to claim 1, wherein the coated quantum dots
obtainable with the method according to claim 1 have a luminescence
maximum wavelength that is shifted relative to a luminescence
maximum wavelength of coated quantum dots obtainable with such
method in the absence of the third element (M3) comprising ion and
fourth element (A4) comprising ion.
7. The method according to claim 1, further comprising embedding
the coated quantum dots in a host material to provide a wavelength
converter element, wherein the host material comprises a
silicone.
8. A luminescent material based on coated quantum dots, the
luminescent material comprising luminescent quantum dots having an
outer layer comprising first element (M1) comprising cations and
second element (A2) comprising anions; and a coating arranged on
said outer layer, wherein the coating comprises a silica coating,
and wherein the silica coating further comprises one or more of a
third element (M3) and a fourth element (A4), wherein the first
element (M1) and the third element (M3) belong to the same group of
the periodic system, and wherein the second element (A2) and the
fourth element (A4) belong to the same group of the periodic
system, and wherein M1 and M3 are each independently selected from
the group consisting of Zn and Cd, and wherein A2 and A4 are each
independently selected from the group consisting of S, Se and
Te.
9. The luminescent material according to claim 8, wherein one or
more of the following applies: M3=M1 and A4=A2.
10. The luminescent material according to claim 8, wherein the one
or more of a third element (M3) and a fourth element (A4) are
available in the coating in an amount of at least 100 ppm,
respectively.
11. The luminescent material according to claim 8, wherein the one
or more of a third element (M3) and a fourth element (A4) are
available in the coating in a weight ratio to silicon in the range
of 1:100-25:100, respectively.
12. The luminescent material according to claim 8, wherein the
quantum dots have a shape selected from the group consisting of a
sphere, a cube, a rod, a wire, a disk, and a multi-pod, wherein the
coating has a thickness (d1) in the range of 1-50 nm, wherein the
quantum dots are of the core-shell type with a core material
differing from the shell material, wherein the core material is
selected from the group consisting of ZnS, ZnSe, CdS, CdSe and InP,
wherein the shell material is selected from the group consisting of
ZnS, ZnSe, CdS, and CdSe, and wherein M3 is selected from the group
consisting of Zn and Cd, and wherein A4 is selected from the group
consisting of S and Se.
13. A wavelength converter element comprising a host material with
the luminescent material, according to claim 8, or obtainable by
the method according to claim 1, embedded therein.
14. The wavelength converter element, wherein the host material
comprises a silicone.
15. A lighting device comprising a light source and a luminescent
material, as defined in claim 8, or a luminescent material
obtainable by the method according to claim 1, or a wavelength
converter element comprising said luminescent material, wherein the
light source is configured to illuminate the luminescent
material.
16. A lamp comprising at least one lighting device according to
claim 15.
17. A luminaire comprising at least one lighting device according
to claim 15 or at least one lamp according to claim 16.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method for the production of a
luminescent material based on quantum dots. The invention further
relates to such luminescent material, and also to a wavelength
converter element comprising such luminescent material. The
invention also relates to a lighting device making use of such
luminescent material to provide light. The invention further
relates to a lamp and a luminaire comprising said lighting
device.
BACKGROUND OF THE INVENTION
[0002] Quantum dots (qdots or QDs) are currently being studied as
phosphors in solid state lighting (SSL) applications (LEDs). They
have several advantages such as a tunable emission and a narrow
emission band which can help to significantly increase the efficacy
of LED based lamps, especially at high CRI. Typically, qdots are
supplied in an organic liquid, with the quantum dots surrounded by
organic ligands, such as oleate (the anion of oleic acid), which
helps to improve the emission efficiency of the dots as well as
stabilize them in organic media.
[0003] The synthesis of silica coatings on quantum dots is known in
the art. Koole et al. (in R. Koole, M. van Schooneveld, J.
Hilhorst, C. de Mello Donega, D.'t Hart, A. van Blaaderen, D.
Vanmaekelbergh and A. Meijerink, Chem. Mater., 20, p. 2503-2512,
2008) describe experimental evidence in favor of a proposed
incorporation mechanism of hydrophobic semiconductor nanocrystals
(or quantum dots, QDs) in monodisperse silica spheres (diameter
.about.35 nm) by a water-in-oil (W/O) reverse microemulsion
synthesis. Fluorescence spectroscopy is used to investigate the
rapid ligand exchange that takes place at the QD surface upon
addition of the various synthesis reactants. It was theorized that
hydrolyzed TEOS has a high affinity for the QD surface and replaces
the hydrophobic amine ligands, which enables the transfer of the
QDs to the hydrophilic interior of the micelles where silica growth
takes place. By hindering the ligand exchange using stronger
binding thiol ligands, the position of the incorporated QDs could
be controlled from centered to off-center and eventually to the
surface of the silica spheres. They were able to make QD/silica
particles with an unprecedented quantum efficiency of 35%.
[0004] WO2013/057702 describes a luminescent nano particles based
luminescent material comprising a matrix of interconnected coated
luminescent nano particles, wherein for instance wherein the
luminescent nano particles comprise CdSe, wherein the luminescent
nano particles comprise a coating of CdS and wherein the matrix
comprises a coating comprising ZnS. The luminescent material
according may have a quantum efficiency of at least 80% at
25.degree. C., and having a quench of quantum efficiency of at
maximum 20% at 100.degree. C. compared to the quantum efficiency at
25.degree. C.
[0005] WO2005/107818 describes fluorescent, radio-opaque and
magnetic quantum nanoparticles, useful as multifunctional contrast
agents or probes for in vivo bio imaging, and methods of their use.
The document describes the use for multifaceted bio imaging (e.g.,
intra-arterial pre-operative brain mapping and broad based in vivo
diagnostic imaging), including imaging of various cell types, such
as stem cells.
[0006] WO2009/046392 describes methods for preparing rare earth
doped monodisperse, hexagonal phase upconverting nanophosphors, the
steps of which include: dissolving one or more rare earth precursor
compounds and one or more host metal fluoride compounds in a
solvent containing a tri-substituted phosphine or a tri-substituted
phosphine oxide to form a solution; heating the solution to a
temperature above about 250.degree. C. at which the phosphine or
phosphine oxide remains liquid and does not decompose; and
precipitating and isolating from the solution phosphorescent
hexagonal phase monodisperse nanoparticles of the host metal
compound doped with one or more rare earth elements. Nanoparticles
and methods for coating rare earth doped upconverting nanophosphors
with SiO.sub.2 are also described.
SUMMARY OF THE INVENTION
[0007] Silica encapsulation of QDs, see also above, is used to
stabilize the QDs in air and to protect them from chemical
interactions with the outside. The reverse micelle method was
introduced in the 90's as a method to make small (.about.20 nm)
silica particles with a small size dispersion (see below). This
method can also be used to make silica-coated QDs. The native
organic ligands around QDs are replaced by inorganic silica
precursor molecules during the silica shell growth. The inorganic
silica shell around QDs has the promise to make QDs more stable
against photo-oxidation, because the organic ligands are seen as
the weak chain in conventional (e.g. oleic acid or hexadecylamine)
capped QDs.
[0008] Though prior art mentions high quantum efficiencies of
quantum dots, especially when coordinated by organic ligands in an
organic medium, it appears that for useful applications where the
quantum dots are provided as particulate material, the quantum
efficiencies dramatically decrease.
[0009] It appeared that the quantum efficiency typically drops by
more than 50% upon silica growth, which makes it not applicable for
lighting applications. In addition, assuming a silica coating,
silica as grown by the reverse micelle method is very porous,
making it a less good barrier against oxygen or water than
sometimes suggested.
[0010] Hence, it is an aspect of the invention to provide an
alternative luminescent material, based on quantum dots, which
preferably further at least partly obviate one or more of
above-described drawbacks. Yet it is also an aspect of the
invention to provide an alternative method for the production of
such luminescent material based on quantum dots. It is further an
aspect of the invention to provide an alternative wavelength
converter and/or alternative lighting device, using such quantum
dots, which preferably further at least partly obviate one or more
of above-described drawbacks.
[0011] Herein, it is proposed to add inorganic salts, such as--but
not limited--to ZnCl.sub.2 or Na.sub.2S, during the shell growth.
This appears to greatly improve the final QE of QDs after shell
growth. Alternatively or in addition, it is proposed to add such
salts after the silica (coating) growth. Also this can improve the
QE and influences peak wavelength and photo brightening effects of
the QDs. With the present method, QD's can be obtained having
efficiencies that are suitable or come close to commercial
applications, such as e.g. in backlighting.
[0012] In a first aspect, the invention provides a method for the
production of a luminescent material based on coated quantum dots
("QDs"), the method comprising: (i) providing luminescent quantum
dots in a liquid medium wherein the luminescent quantum dots have
an outer layer comprising first element (M1) comprising cations and
second element (A2) comprising anions; and (ii) providing in a
coating process a coating on the outer layer of the quantum dots in
the liquid medium, wherein the coating especially comprises a
silica coating; wherein during the coating process, or after the
coating process, or during and after the coating process, the
liquid medium comprises one or more of a third element (M3)
comprising ion and a fourth element (A4) comprising ion, wherein in
a specific embodiment the first element (M1) and the third element
(M3) belong to the same group of the periodic system, and wherein
in a specific embodiment the second element (A2) and the fourth
element (A4) belong to the same group of the periodic system.
Especially, the first element comprising ions are metal cations,
such as divalent zinc. Further, especially the second element
comprising ions are non-metal anions, such as sulfide or selenide.
Note that the term element herein does not indicate that elemental
material is applied (like e.g. the Zn element/metal or the S
element). In general the elements will be available or will be
provided as ions comprising such element(s), like Zn.sup.2+,
Cd.sup.2+, or S.sup.2-, Se.sup.2-, Te.sup.2-, etc.
[0013] The invention also provides in a further aspect the
luminescent material, obtainable with such method. Hence, in (yet)
a further aspect, the invention also provides luminescent material
based on coated quantum dots, the luminescent material comprising
luminescent quantum dots having an outer layer comprising first
element (M1) comprising cations and second element (A2) comprising
anions; and a coating arranged on said outer layer, wherein the
coating especially comprises a silica coating, and wherein the
(silica) coating further comprises one or more of a third element
(M3) and a fourth element (A4), wherein the first element (M1) and
the third element (M3) may in an embodiment belong to the same
group of the periodic system, and wherein the second element (A2)
and the fourth element (A4) may in an embodiment belong to the same
group of the periodic system. Especially, the coating comprises a
silica (SiO.sub.2) coating. Alternatively or additionally, the
coating may comprise a titania (TiO.sub.2) coating, an alumina
(Al.sub.2O.sub.3) coating, or a zirconia (ZrO.sub.2) coating.
Especially, here below the invention is described with respect to a
coating comprising a silica coating.
[0014] With this method, quantum dot based luminescent material was
obtained in the lab with quantum efficiencies even well above 70%
(in air). Further, also quantum dots embedded in matrices in
practice did not appear to have such high QE's. The present method
seems to provide for the first time particulate quantum dots and/or
quantum dots in matrices having interestingly high quantum
efficiencies. It is theorized that, assuming a zinc sulfide outer
layer at the QD (such as when assuming a ZnSe quantum dot, or a CdS
core/ZnS shell quantum dot), zinc-containing ligand and/or
sulphur-containing ligand complexes may leave the QD surface during
silica shell growth, causing trap states that reduce QE. By adding
e.g. zinc and/or sulphide salts during and/or after silica shell
growth, these traps states may be recovered. Hence, the one or more
of a third element (M3) comprising ion and a fourth element (A4)
comprising ion are especially applied to improve the quantum
efficiency and/or stability. Further, it surprisingly appears that
the addition of salts may not only increase QE, but also stability
against high temperature or blue flux improves.
[0015] Quantum dots may be provided as bare particles, or may e.g.
be provided as core-shell particles. The term "shell" may also
refer to a plurality of shells. Further, core-shell particles are
not necessarily spherical; they may e.g. also be of the quantum rod
type or tetrapod type (or other multipod type), etc. Further
examples are provided below. The bare particle or core is
especially the optically active part. The shell is used as a kind
of protection and often comprises a similar type of material, such
as a ZnSe core and a ZnS shell (see also below). Such particles are
commercially available in organic liquids, with organic ligands
attached to such particles for better dispersion. Herein, the outer
layer of the particle is the layer most remote from a central part
of the bare particle or the core. In the case of a ZnS shell, this
outer layer would be the ZnS surface of the QD. The invention is,
however, not limited to quantum dots whit a ZnS shell and a ZnSe
core. Below, a number of alternative quantum dots are described.
Hence, the term "quantum dot" especially refers to a quantum dot
particle, which may thus be a bare particle, or a core shell
particle, and which may comprise a dot, a rod, etc. As known in the
art, a rod in general has a quantum dot incorporated in a rod
structure, such as a ZnSe dot in a ZnS rod. Hence, in the case of a
"simple" ZnSe or CdS quantum dot, the outer layer will
substantially comprise ZnSe and CdS, respectively. Would however
core-shell QDs be applied, such as ZnSe with a ZnS shell, or CdS
with a ZnS shell, then the outer layer will substantially comprise
ZnS (in both embodiments).
[0016] Therefore, in an embodiment wherein the quantum dots are not
of the core-shell type, but essentially consist of a single
material without a shell, the core is of the M1-A2 type, i.e. the
core comprises M1 and A2, wherein M1-A2 especially represent any
relevant semiconducting material that can be used as QD, see e.g.
all examples below. Hence, here the outer layer is substantially
identical to the remainder of the QD (core).
[0017] In case the QD is of the core-shell type, both the core and
the shell may comprise M1-A2, but either M1 or A2, or both M1 and
A2 of the shell differ from M1 and A2 of the core. For instance, a
ZnSe core with a ZnS shell. Here, the shell is the outer layer.
Hence, M1 and A2 refer to the cation(s) and anion(s), respectively,
of the outer layer.
[0018] On such an outer layer, a (silica) coating may be provided,
thereby providing a bare quantum dot with a (silica) coating or a
core-shell quantum dot with a (silica) coating. Coating quantum
dots with silica results in replacement of the organic ligands by
silica precursor molecules, which may act as more (light and/or
temperature) stable inorganic ligands. In addition, the silica
layer may form a protective barrier against e.g. photo-oxidative
species. Especially, the coating entirely covers the outer layer.
Suitable methods to provide silica coatings around QDs are amongst
others described by Koole et al. (see above), and references cited
therein. The synthesis of silica particles without nanoparticles
enclosed was first developed by Stober et al (J. Colloid Interface
Sci. 1968, 62), which allows the growth of silica spheres of
uniform size and shape in e.g. an ethanol phase. The second method
of making silica spheres uses micelles in an apolar phase and is
called the reverse micelle method (or reverse micro emulsion
method), and was first suggested by Osseo-Asare, J. Colloids. Surf.
1990, 6739). The silica particles are grown in defined water
droplets, which results in a uniform size distribution which can be
controlled quite easily. This approach was extended by introducing
nanoparticles in the silica. The main advantage of this approach
compared to the Stober method, is that both hydrophobic and
hydrophilic particles can be coated, no ligand exchange on forehand
is required and there is more control over the particles size and
size dispersion.
[0019] The present invention is not limited to one of these
methods. However, in a specific embodiment the coating process is
executed in micelles containing said quantum dots, especially using
the reverse-micelle method, as also discussed in Koole et al.,
which is herein incorporated by reference. Hence, the coating
process is especially a process wherein the coating, especially an
oxide coating, even more especially a silica coating, is provided
to the outer layer of the QD, and which coating process is
especially performed in micelles, wherein the QD is exposed to the
(silica) precursor with the aid of micelles (containing e.g. water
and ammonia as a non-continuous phase in the continuous (non-polar)
liquid medium), to provide said silica coating. A micelle may
especially be defined as an aggregate of surfactant molecules
dispersed in a liquid medium. A typical micelle in aqueous solution
forms an aggregate with the hydrophilic "head" regions in contact
with surrounding solvent, sequestering the hydrophobic single-tail
regions in the micelle center. A reverse micelle is the other way
around, using an apolar solution and where the hydrophilic "heads"
are pointing inwards and the hydrophobic tail regions are in
contact with the apolar medium. As indicated elsewhere, instead of
silica, also other coatings may be provided using a coating
precursor for such coating.
[0020] As indicated above, the availability of the cations and/or
anions, other than those of the silica/silicate appears to have a
positive influence on quantum efficiency and/or stability of the
QDs. It appears that the use of cations and anions similar to those
used in the outer shell of the original qdots is most beneficial.
For instance, when using a ZnS shell quantum dot, the use of Zn
and/or S ions appeared to be beneficial whereas the use of Na (or
K) and Cl as ions appeared to have no positive effect.
[0021] Hence, especially the first element (M1) and the third
element (M3) belong to the same group of the periodic system and/or
the second element (A2) and the fourth element (A4) belong to the
same group of the periodic system. This phrase does not imply that
in each embodiment both a similar cation and a similar anion as QE
improver material have to be applied. As indicated above, during
the coating process, or after the coating process, or during and
after the coating process, the liquid medium may comprise one or
more of a third element (M3) comprising ion and a fourth element
(A4) comprising ion. For instance, assuming again a ZnS outer
layer, only ZnCl.sub.2 may be applied during and/or after the
coating process, or only Na.sub.2S may be applied during and/or
after the coating process.
[0022] Further, especially the third element and/or fourth element
are selected from the periods 3-5, even more especially 4-5, and
especially, when M3.noteq.M1 and A4.noteq.A2, M3 and/or A4 are
selected from a period above (if any) or a period below (if any) of
M1 and A2, respectively. For instance, again assuming a ZnS outer
layer, M3 may especially be Cd and/or A4 may especially be Se. The
terms third element and fourth element may in embodiments
(independently) also refer to a combination of different third
elements and a combination of different fourth elements,
respectively.
[0023] Hence, in a specific embodiment one or more of the following
applies: M3=M1 and A4=A2, as this may especially give good results.
The terms "third element" or "fourth element" may also refer to a
plurality of different third elements or fourth elements,
respectively. For instance, assuming a ZnS outer layer, the third
element may for instance include one or more of the zinc cation and
the cadmium cation, and the fourth element may for instance include
one or more of the sulphide anion and the selenide or telluride
anion, respectively. In such embodiments, the fourth element (A4)
comprising ion is selected from e.g. the group consisting of the
sulphide anion and the selenide or telluride anion. Likewise, the
terms "third element (M3) comprising ion" and "fourth element (A4)
comprising ion" may in embodiments (independently) also refer to a
combination of different third element (M3) comprising ions or a
combination of different fourth element (A4) comprising ions,
respectively.
[0024] In yet a further specific embodiment, M1 and M3 are each
independently selected from the group consisting of Zn and Cd, and
A2 and A4 are each independently selected from the group consisting
of S, Se and Te, more especially independently selected from the
group consisting of S and Se. This may e.g. correspond to QDs
essentially consisting of ZnS, ZnSe, CdS, CdSe, etc. This may also
correspond to core-shell quantum dots, wherein the shell
essentially consist of ZnS, ZnSe, CdS, CdSe, etc.; the core may
e.g. comprise CdS, ZnSe, InP, etc. Note that when core-shell QDs
are described, the chemical composition of the core and the shell
will especially be different.
[0025] The coating process can e.g. be stopped when the coated
quantum dots are precipitated or removed from the liquid medium
(such as cyclohexane). Further, the coating process may
self-terminate when the coating process is substantially ready.
This may sometimes take days. Thereafter, the quantum dots can be
washed with (another) liquid medium. In principle, the treatment
with the quantum efficiency enhancing ions may also be executed
with the washing liquid, or after washing, in another liquid.
However, especially the treatment with one or more of these ions is
executed before the washing procedure, i.e. in general in the same
liquid medium as wherein the coating or silica growth process is
executed. This liquid medium, wherein the coating process is
executed (in the micelle), is especially an apolar organic solvent,
such as pentane, hexane, heptane, etc., including any isomer
thereof, such as cyclohexane, etc. In the apolar liquid medium,
micelles are formed by nonionic or ionic surfactants, enclosing
small water/ammonia droplets. The oxide, especially silica, growth
takes place within these small water droplets. Further, as
indicated above during the coating process (such as in the
micelle), or after the coating process, or during and after the
coating process, the liquid medium may comprise one or more of a
third element (M3) comprising ion and a fourth element (A4)
comprising ion. In a specific embodiment, good results can be
obtained when during the coating process the liquid medium
comprises the third element (M3) comprising ion and does
substantially not comprise the fourth element (A4) comprising ion,
and wherein after the coating process the liquid medium comprises
the fourth element (A4) comprising ion.
[0026] However, the combination of salts and/or the sequence of the
salts may also depend upon the type of chemicals and liquids used
during the coating process. The third element (M3) comprising ion
and fourth element (A4) comprising ion are added to the growth
medium typically as a solution in water and will therefore be part
of the small water droplets surrounded by the surfactants, in which
silica growth takes place. Optionally, before introduction of the
fourth element, the liquid medium may be replaced with another
liquid medium (substantially not comprising the third element). In
other embodiments, wherein during the coating process the liquid
medium comprises the fourth element and does substantially not
comprise the third element, and wherein after the coating process
the liquid medium comprises the third element, also before
introduction of the third element, the liquid medium may be
replaced with another liquid medium (substantially not comprising
the fourth element).
[0027] The liquid medium wherein the quantum dots are available may
thus contain the third element and/or the fourth element (such as
in the micelles in said liquid medium). The concentration of the
third element and/or the fourth element may be up to saturation of
the respective salts that provide the third element and/or the
fourth element, respectively. Further, the concentration should
especially not be too low to have an effect, especially in case
when the second ion(s) are applied after the start of the coating
process, or even after the (substantial) termination of the coating
process. Hence, in a specific embodiment the one or more of the
third element (M3) and the fourth element (A4) are available in the
liquid medium in a concentration of at least of 5 mM, such as at
least 10 mM, such as at least 20 mM, or even higher, such as at
least 0.05 M. Hence, the invention includes a deliberate addition
of one or more salts during and/or after the coating process.
[0028] It surprisingly appears that the presence of the quantum
efficiency enhancing elements (or ions) has not only an effect on
the quantum efficiency, but also on the position of the emission
wavelength. In general, it appears that the ion induces a shift,
with the third element tending to give a red shift, and the fourth
element tending to give a blue shift of the emission. The shift of
the emission relative to the untreated silica coated QDs can be
indicative of the specific process described herein. Hence, in a
specific embodiment the coated quantum dots obtainable with the
method according to any one of the preceding claims have a
luminescence maximum wavelength that is shifted relative to a
luminescence maximum wavelength of coated quantum dots obtainable
with such method in the absence of the third element (M3)
comprising ion and fourth element (A4) comprising ion. Further, as
indicated above the presence of the salts during or after the
coating process also improves stability against high temperature or
blue flux.
[0029] After the QDs have been coated and treated with the anions
and/or cations, the QDs can be washed. Optionally, they can be
redispersed in a liquid ((second) liquid medium), or can be used as
such (see also below).
[0030] Further, it surprisingly appears that when the liquid medium
comprises the fourth element (A4), such as S, Se and/or Te, it may
be advantageous to use a basic medium, especially also comprising
one or more of NaOH and KOH. Hence, when the fourth element is
applied, especially an liquid medium may be applied that is an
aqueous medium, such as water comprising Na.sub.2S and NaOH (which
will be dissolved, i.e. Na.sup.+, S.sup.2-, and OH.sup.-).
[0031] The pH used of the basic liquid medium especially be 11 or
higher, such as 12 or higher, like even 13 or higher. Especially,
the pH of the liquid the particle may experience is (also) at least
about 11, such as at least about 12, or even at least about 13.
[0032] After the coating (and after the optional washing), the
coated quantum dots may in a first embodiment be separated from the
liquid material. This can be done with methods known in the art,
like filtration, evaporation, etc. The luminescent material
obtainable with the method as described herein comprises
particulate coated quantum dots. This material may be dried and be
converted into particulate material (comprising coated quantum
dots). Alternatively, in a second embodiment the coated quantum
dots can be redispersed in a liquid ((second) liquid medium). Both
materials may be used for embedding the coated quantum dots in a
matrix (see below).
[0033] Especially however, the method as defined herein may further
comprise embedding the coated quantum dots in a host material to
provide a wavelength converter element, wherein the host material
may especially comprise a silicone (poly siloxanes polymers).
However, other host materials may also be possible. Hence, the
invention also provides a wavelength converter element comprising a
host material with the luminescent material, as described herein,
or obtainable by the method according as described herein, embedded
therein.
[0034] Especially, the host material is transmissive for light,
especially UV and/or blue light. The wavelength converter element
is especially a transmissive element and may also be indicated as
waveguide or light guide. This host material may especially be a
transmissive host material, and may be of inorganic or organic
character. For instance, the host material may comprise one or more
materials selected from the group consisting of PE (polyethylene),
PP (polypropylene), PEN (polyethylene napthalate), PC
(polycarbonate), polymethylacrylate (PMA), polymethylmethacrylate
(PMMA) (Plexiglas or Perspex), cellulose acetate butyrate (CAB),
polyvinylchloride (PVC), polyethylene terephthalate (PET), (PETG)
(glycol modified polyethylene terephthalate), PDMS
(polydimethylsiloxane), PMPS (polymethylphenylsiloxane), and COC
(cyclo olefin copolymer). However, in another embodiment the
waveguide may comprise an inorganic material. Preferred (in)organic
materials are selected from the group consisting of glasses,
(fused) quartz, transmissive ceramic materials, and silicones. Also
hybrid materials, comprising both inorganic and organic parts may
be applied. Especially preferred are PMMA, transparent PC,
silicone, or glass as material for the waveguide. Especially,
silicone may be of interest, but also PDMS, PMPS, and
polysilsesquioxane. Especially, the host material is transmissive
for light having a wavelength selected from the range of 380-750
nm. For instance, the host material may be transmissive for blue,
and/or green, and/or red light. Especially, the host material is
transmissive for at least the entire range of 420-680 nm.
Especially, the host material has a light transmission in the range
of 50-100%, especially in the range of 70-100%, for light generated
by the light source of the lighting unit (see also below) and
having a wavelength selected from the visible wavelength range. In
this way, the host material is transmissive for visible light from
the lighting unit. The transmission or light permeability can be
determined by providing light at a specific wavelength with a first
intensity to the material and relating the intensity of the light
at that wavelength measured after transmission through the
material, to the first intensity of the light provided at that
specific wavelength to the material (see also E-208 and E-406 of
the CRC Handbook of Chemistry and Physics, 69th edition,
1088-1989). The light converter may be transparent or translucent,
but may especially be transparent. Especially, the light converter
is substantially transparent and/or does not substantially scatter
light. When the light converter is transparent, light of the light
source may not entirely be absorbed by the light converter.
Especially when using blue light, this may be of interest, as the
blue light may be used to excite the QDs or light converter nano
particles and may be used to provide a blue component (in white
light). Hence, especially curable siloxane polymers are applied
that provide a substantially transmissive matrix (or host) for the
QDs or light converter nano particles.
[0035] In yet another embodiment, the host material comprises a
salt that is obtainable by coprecipitation of precursor salts in
the presence of the quantum dots.
[0036] Further, the wavelength converter element may be used as
such. However, the host material enclosing the quantum dots may
also be processed (again) into particulate material. This
particulate luminescent material may be used in a luminescent
application wherein e.g. particles are coated as layer or are
embedded, e.g. in a silicone matrix or other dome material of a LED
dome, etc. Optionally, the host material may include further
luminescent materials which may be based on (other) quantum dots or
other type of luminescent materials, like oxide or nitride based
luminescent materials.
[0037] As indicated above, the luminescent material obtainable with
the herein described method may be used as such, such as a
particulate material, or in a liquid (dispersion or colloid) or in
a wavelength converter element. It especially appears that the thus
obtained quantum dots have a silica coating wherein the third
elements and/or fourth elements can be traceable. They may be
embedded as cation or anion, respectively. Especially, the one or
more of a third element (M3) comprising ion and a fourth element
(A4) comprising ion are available in the coating in an amount of at
least 50 ppm, such as at least 100 ppm, especially at least 200
ppm, respectively. For instance, when ZnCl.sub.2 is applied, e.g.
10 ppm of Zn may be found in the coating (and optionally also Cl).
Likewise, when Na.sub.2S is applied, e.g. 100 ppm of S may be found
(and e.g. also 10 ppm of Na) in the coating. Here ppm especially
refers to atoms of the indicated specie(s) relative to the total
number of atoms of the coating (i.e. especially Si and O in the
case of a silica coating). Hence, the term "ppm" especially refers
to "atom ppm". In a specific embodiment, a luminescent material is
provided wherein the one or more of a third element M3 and a fourth
element A4 are available in the coating in a weight ratio to
silicon in the range of 1:100-25:100, respectively, such as
2:100-25:100, respectively. Hence, the coating may e.g. comprise Zn
in such a weight percentage, that a Zn:Si weight ratio in the range
of 1:100-25:100 is provided. Likewise, the coating may e.g.
comprise S in such a weight percentage, that a S:Si weight ratio in
the range of 1:100-25:100 is provided. In a specific embodiment,
the third element M3 is available in the coating in a weight ratio
to silicon in the range of 1:100-25:100 and the fourth element A4
is available in the coating in a weight ratio to silicon in the
range of 1:100-25:100. When M3 and/or A4 comprise a combination of
elements, such as e.g. in an embodiment Zn and Cd and/or in another
embodiment Se and S, respectively, the weight ratio range may
especially apply the weight of the combination of elements. The
coating thus comprises silica, and in addition the coating
comprises one or more of M3 and A4, especially M3 in the form of a
M3 oxide and A3 in the form of a sodium or other alkaline earth A4
species. M3 and/or A4 may be embedded in the silica, or may be at
an interface of the outer layer of the quantum dot, or may at an
outer layer of the silica layer. Also combinations may be possible.
The distribution of M3 and/or A4 over the silica comprising layer
may also depend when and how M3 and/or A4 are applied during the
synthesis process.
[0038] When the third element and/or fourth element are applied
after the coating process has commenced, or especially after the
coating process substantially terminated, there may be a
concentration gradient of the third element and/or fourth element,
respectively, with a higher concentration more remote from the QD
and a lower concentration closer to the QD.
[0039] Amongst others, the cations and/or anions may be detected
with TEM, EDX, or total reflection x-ray fluorescence in
conjunction with synchrotron radiation (SR-TXRF). Further, by
etching or dissolving thin layers, one may perform elemental
analysis on the etched or dissolved material such as by ICP-MS.
Especially when one or more of M3.noteq.M1 and A4.noteq.A2 applies,
detection of the presence of an element that has been used during
and/or after the coating process may be easy, as this may be an
element that does not naturally belong to the outer coating and
neither naturally belong to the (silica) coating.
[0040] The (final) (silica comprising) coating may e.g. have a
thickness in the range of 1-50 nm, such as 2-20 nm. Further, the
herein described (silica) coating process, which is executed in the
presence of the third element and/or fourth element and/or which is
followed by a treatment or impregnation with the third element
and/or fourth element, may optionally be subjected to a further
coating process and/or embodiment in a host material (see further
herein).
[0041] The luminescent material may in an embodiment comprise a
liquid comprising said silica coated quantum dots, optionally with
capping agent coordinating to the quantum dots. This luminescent
material may be a dispersion or colloid or gel. Applications of
such luminescent material may include lighting application wherein
the luminescent material is enclosed in a vessel or cuvette like
body or another envelope. However, the luminescent material when
dissolved in an aqueous liquid may also be used for biological
applications, including medical applications, for instance as
biomarkers. Other options include photovoltaic applications or
photodiode applications.
[0042] In yet another embodiment, the luminescent material
substantially comprises the QDs. For instance, the QDs may be
separated from the liquid with techniques known in the art,
including evaporation of the liquid, etc., thereby providing the
QDs agents as powder or cake. Subsequently, the thus obtained
material may be further processed (see also below) into e.g.
particulate material. For instance, the luminescent material may
also be provided as coating on a substrate. The luminescent
material substantially comprising the QDs may also be encapsulated
in a matrix, such as an inorganic or organic matrix, to provide
e.g. a wavelength converter element.
[0043] The quantum dots or luminescent nanoparticles, which are
herein indicated as wavelength converter nanoparticles, may for
instance comprise group II-VI compound semiconductor quantum dots
selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe,
ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe,
HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe,
HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS,
CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe and HgZnSTe. In another
embodiment, the luminescent nanoparticles may for instance be group
III-V compound semiconductor quantum dots selected from the group
consisting of GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, GaNP,
GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs, GaAlNP,
GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, and
InAlPAs. In yet a further embodiment, the luminescent nanoparticles
may for instance be I-III-VI2 chalcopyrite-type semiconductor
quantum dots selected from the group consisting of CuInS.sub.2,
CuInSe.sub.2, CuGaS.sub.2, CuGaSe.sub.2, AgInS.sub.2, AgInSe.sub.2,
AgGaS.sub.2, and AgGaSe.sub.2. In yet a further embodiment, the
luminescent nanoparticles may for instance be I-V-VI2 semiconductor
quantum dots, such as selected from the group consisting of
LiAsSe.sub.2, NaAsSe.sub.2 and KAsSe.sub.2. In yet a further
embodiment, the luminescent nanoparticles may for instance be a
group IV-VI compound semiconductor nano crystals such as SbTe. In a
specific embodiment, the luminescent nanoparticles are selected
from the group consisting of InP, CuInS.sub.2, CuInSe.sub.2, CdTe,
CdSe, CdSeTe, AgInS.sub.2 and AgInSe.sub.2. In yet a further
embodiment, the luminescent nanoparticles may for instance be one
of the group II-VI, III-V, I-III-V and IV-VI compound semiconductor
nano crystals selected from the materials described above with
inside dopants such as ZnSe:Mn, ZnS:Mn. The dopant elements could
be selected from Mn, Ag, Zn, Eu, S, P, Cu, Ce, Tb, Au, Pb, Tb, Sb,
Sn and Tl. Herein, the luminescent nanoparticles based luminescent
material may also comprise different types of QDs, such as CdSe and
ZnSe:Mn.
[0044] It appears to be especially advantageous to use II-VI
quantum dots. Hence, in an embodiment the semiconductor based
luminescent quantum dots comprise II-VI quantum dots, especially
selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe,
ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe,
HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe,
HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS,
CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe and HgZnSTe, even more
especially selected from the group consisting of CdS, CdSe,
CdSe/CdS and CdSe/CdS/ZnS. In an embodiment, however, Cd-free QDs
are applied. In a specific embodiment, the wavelength converter
nano-particles comprise III-V QDs, more specifically an InP based
quantum dots, such as a core-shell InP--ZnS QDs. Note that the
terms "InP quantum dot" or "InP based quantum dot" and similar
terms may relate to "bare" InP QDs, but also to core-shell InP QDs,
with a shell on the InP core, such as a core-shell InP--ZnS QDs,
like a InP--ZnS QDs dot-in-rod.
[0045] Hence, more in general M1 and M3 are each independently
selected from the group consisting of Zn, Cd, and Hg, and A2 and A4
are each independently selected from the group consisting of S, Se
and Te, more especially independently selected from the group
consisting of S and Se. This may especially be relevant for II-VI
based quantum dots, or for core-shell type quantum dots comprising
an outer layer comprising such II-VI material.
[0046] Alternatively or additionally, M1 and M3 are each
independently selected from the group consisting of Ga, Al and In,
and A2 and A4 are each independently selected from the group
consisting of P, N and As. This may especially be relevant for
III-V based quantum dots, or for core-shell type quantum dots
comprising an outer layer comprising such III-V material.
[0047] Alternatively or additionally, M1 and M3 are each
independently selected from the group consisting of Cu, Ga, Ag and
In, and A2 and A4 are each independently selected from the group
consisting of S, Se and Te, more especially independently selected
from the group consisting of S and Se. This may especially be
relevant for I-III-VI2 based quantum dots, or for core-shell type
quantum dots comprising an outer layer comprising such I-III-VI2
material.
[0048] Alternatively or additionally, M1 and M3 are each
independently selected from the group consisting of Li, Na, and K,
in combination with As, and A2 and A4 are each independently
selected from the group consisting of S, Se and Te, more especially
independently selected from the group consisting of S and Se, even
more especially Se. This may especially be relevant for I-V-VI2
based quantum dots, or for core-shell type quantum dots comprising
an outer layer comprising such I-V-VI2 material.
[0049] The luminescent nanoparticles (without coating) may have
dimensions in the range of about 1-50 nm, especially 1-20 nm, such
as 1-15 nm, like 1-5 nm; especially at least 90% of the
nanoparticles have dimension in the indicated ranges, respectively,
(i.e. e.g. at least 90% of the nanoparticles have dimensions in the
range of 2-50 nm, or especially at least 90% of the nanoparticles
have dimensions in the range of 5-15 nm). The term "dimensions"
especially relate to one or more of length, width, and diameter,
dependent upon the shape of the nanoparticle. In an embodiments,
the wavelength converter nanoparticles have an average particle
size in a range from about 1 to about 1000 nanometers (nm), and
preferably in a range from about 1 to about 100 nm. In an
embodiment, nanoparticles have an average particle size in a range
from about 1-50 nm, especially 1 to about 20 nm, and in general at
least 1.5 nm, such as at least 2 nm. In an embodiment,
nanoparticles have an average particle size in a range from about 1
to about 20 nm.
[0050] Typical dots may be made of binary alloys such as cadmium
selenide, cadmium sulfide, indium arsenide, and indium phosphide.
However, dots may also be made from ternary alloys such as cadmium
selenide sulfide. These quantum dots can contain as few as 100 to
100,000 atoms within the quantum dot volume, with a diameter of 10
to 50 atoms. This corresponds to about 2 to 10 nanometers. For
instance, (spherical) particles such as CdSe, InP, or CuInSe.sub.2,
with a diameter of about 3 nm may be provided. The luminescent
nanoparticles (without coating) may have the shape of spherical,
cube, rods, wires, disk, multi-pods, etc., with the size in one
dimension of less than 10 nm. For instance, nanorods of CdSe with
the length of 20 nm and a diameter of 4 nm may be provided. Hence,
in an embodiment the semiconductor based luminescent quantum dots
comprise core-shell quantum dots. In yet another embodiment, the
semiconductor based luminescent quantum dots comprise dots-in-rods
nanoparticles. A combination of different types of particles may
also be applied. For instance, core-shell particles and
dots-in-rods may be applied and/or combinations of two or more of
the afore-mentioned nano particles may be applied, such as CdS and
CdSe. Here, the term "different types" may relate to different
geometries as well as to different types of semiconductor
luminescent material. Hence, a combination of two or more of (the
above indicated) quantum dots or luminescent nano-particles may
also be applied. Hence, in an embodiment the quantum dots have a
shape selected from the group consisting of a sphere, a cube, a
rod, a wire, a disk, and a multi-pod, etc. A combination of
different types of particles may also be applied. Here, the term
"different types" may relate to different geometries as well as to
different types of semiconductor luminescent material. Hence, a
combination of two or more of (the above indicated) quantum dots or
luminescent nano-particles may also be applied.
[0051] In an embodiment, nanoparticles or QDs can comprise
semiconductor nanocrystals including a core comprising a first
semiconductor material and a shell comprising a second
semiconductor material, wherein the shell is disposed over at least
a portion of a surface of the core. A semiconductor nanocrystal or
QD including a core and shell is also referred to as a "core/shell"
semiconductor nanocrystal.
[0052] For example, the semiconductor nanocrystal or QD can include
a core having the formula MX, where M can be cadmium, zinc,
magnesium, mercury, aluminum, gallium, indium, thallium, or
mixtures thereof, and X can be oxygen, sulfur, selenium, tellurium,
nitrogen, phosphorus, arsenic, antimony, or mixtures thereof.
Examples of materials suitable for use as semiconductor nanocrystal
cores include, but are not limited to, ZnO, ZnS, ZnSe, ZnTe, CdO,
CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe, GaSb, HgO, HgS,
HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, TIN, TIP,
TlAs, TlSb, PbO, PbS, PbSe, PbTe, Ge, Si, an alloy including any of
the foregoing, and/or a mixture including any of the foregoing,
including ternary and quaternary mixtures or alloys.
[0053] The shell can be a semiconductor material having a
composition that is the same as or different from the composition
of the core. The shell comprises an overcoat of a semiconductor
material on a surface of the core semiconductor nanocrystal can
include a Group IV element, a Group II-VI compound, a Group II-V
compound, a Group III-VI compound, a Group III-V compound, a Group
IV-VI compound, a Group I-III-VI compound, a Group II-IV-VI
compound, a Group II-IV-V compound, alloys including any of the
foregoing, and/or mixtures including any of the foregoing,
including ternary and quaternary mixtures or alloys. Examples
include, but are not limited to, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,
CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe, GaSb, HgO, HgS, HgSe,
HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, TIN, TIP, TlAs,
TlSb, PbO, PbS, PbSe, PbTe, Ge, Si, an alloy including any of the
foregoing, and/or a mixture including any of the foregoing. For
example, ZnS, ZnSe or CdS overcoatings can be grown on CdSe or CdTe
semiconductor nanocrystals. An overcoating process is described,
for example, in U.S. Pat. No. 6,322,901. By adjusting the
temperature of the reaction mixture during overcoating and
monitoring the absorption spectrum of the core, over coated
materials having high emission quantum efficiencies and narrow size
distributions can be obtained. The overcoating may comprise one or
more layers. The overcoating comprises at least one semiconductor
material which is the same as or different from the composition of
the core. Preferably, the overcoating has a thickness from about
one to about ten monolayers. An overcoating can also have a
thickness greater than ten monolayers. In an embodiment, more than
one overcoating can be included on a core.
[0054] In an embodiment, the surrounding "shell" material can have
a band gap greater than the band gap of the core material. In
certain other embodiments, the surrounding shell material can have
a band gap less than the band gap of the core material. In an
embodiment, the shell can be chosen so as to have an atomic spacing
close to that of the "core" substrate. In certain other
embodiments, the shell and core materials can have the same crystal
structure. Examples of semiconductor nanocrystal (core)shell
materials include, without limitation: red (e.g., (CdSe)ZnS
(core)shell), green (e.g., (CdZnSe)CdZnS (core)shell, etc.), and
blue (e.g., (CdS)CdZnS (core)shell (see further also above for
examples of specific wavelength converter nanoparticles, based on
semiconductors. Herein, the terms "semiconductor nanocrystal" and
"QD" are used interchangeably.
[0055] Hence, the above-mentioned outer surface may be the surface
of a bare quantum dot (i.e. a QD not comprising a further shell or
coating) or may be the surface of a coated quantum dot, such as a
core-shell quantum dot (like core-shell or dot-in-rod), i.e. the
(outer) surface of the shell. The grafting ligand thus especially
grafts to the outer surface of the quantum dot, such as the outer
surface of a dot-in-rod QD.
[0056] Therefore, in a specific embodiment, the wavelength
converter nanoparticles are selected from the group consisting of
core-shell nano particles, with the cores and shells comprising one
or more of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe,
CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe,
CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe,
HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe,
HgZnSeS, HgZnSeTe, HgZnSTe, GaN, GaP, GaAs, AlN, AlP, AlAs, InN,
InP, InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs,
InPAs, GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP,
InAlNAs, and InAlPAs. In general, the cores and shells comprise the
same class of material, but essentially consist of different
materials, like a ZnS shell surrounding a CdSe core, etc.
[0057] The term wavelength converter refers to a system that is
configured to convert light from a first wavelength into light of a
second wavelength. Especially, UV and/or blue light (excitation
wavelength) may be (at least partially) converted into (visible)
light of higher wavelength than the excitation wavelength. This
will further be elucidated below.
[0058] The wavelength converter element may e.g. be (configured as)
a layer, such as a film, especially a polymeric layer, or a body,
such as a dome. Alternatively or additionally, the wavelength
converter may also be (configured as) a lens or reflector.
[0059] Hence, especially the first element (M1) and the third
element (M3) are (independently) selected from the group of metals,
such as especially selected from the group of Cd, Zn, Hg, Ga, In
Al, Tl, Pb, Cu, Ge, Sn, V, especially from the group consisting of
Cd, Zn, Hg, Ga, In, and Al. Especially, the first element (M1)
comprising ion comprises a cation, and is especially selected from
the cations of the elements selected from the group consisting of
Cd, Zn, Hg, Ga, In, Al, Tl, Pb, Cu especially from the group
consisting of Cd, Zn, Hg, Ga, In, and Al.
[0060] Further, especially the second element (A2) and the fourth
element (A4) are (independently) selected from the group of
non-metals, such as especially selected from the group consisting
of S, Se, Te, N, P, Sb, and As, especially from the group
consisting of S, Se, Te, N, and P, such as S and/or Se. Especially
the second element (A1) comprising ion comprises an anion, and is
especially selected from anions of the elements selected from the
group consisting of S, Se, Te, N, P, and As.
[0061] The outer layer may especially be a compound essentially
based on M1 and A2, such as a semiconductor outer layer like ZnS,
where M1=Zn and A2=S.
[0062] Note that in e.g. systems like HgSeTe A2 may refer to a
combination of two elements. Hence, M1, A2, M3, and A4
independently may include one or more of the herein indicated
elements for these categories.
[0063] In yet a further specific embodiment, the first element (M1)
and the third element (M3) do not belong to the same group of the
periodic system and/or the second element (A2) and the fourth
element (A4) belong to the same group of the periodic system. For
instance, in an embodiment the outer layer may comprise ZnS and the
fourth element comprises P.
[0064] In yet a further aspect of the invention, it appears that
especially SO.sub.4.sup.2- and/or PO.sub.4.sup.3-, substantially
irrespective of the outer layer, may have a QE increasing and/or
stabilizing effect (on the life time). Hence, in yet a further
embodiment the invention also provides method for the production of
a luminescent material based on coated quantum dots, the method
comprising: (i) providing luminescent quantum dots in a liquid
medium wherein the luminescent quantum dots have an outer layer;
and (ii) providing in a coating process a coating on the outer
layer of the quantum dots in the liquid medium, wherein the coating
comprises a silica coating; wherein during the coating process, or
after the coating process, or during and after the coating process,
the liquid medium comprises one or more of SO.sub.4.sup.2- and
PO.sub.4.sup.3-. Other ions that may be applied may e.g. include
one or more of the aluminate ion (Al(OH).sub.4.sup.-), the stannate
ion (SnO.sub.3.sup.-, SnO.sub.3.sup.2-, and SnO.sub.4.sup.4-), the
vanadate ion (VO.sub.3.sup.-, VO.sub.4.sup.3-), the molybdate ion
(MoO.sub.4.sup.2-), the tungstate ion (WO.sub.4.sup.2) and the
zincate ion (Zn(OH).sub.4.sup.2-), and other complex metal ions.
These may also be used in combination of ions wherein a
corresponding element is used (i.e. an element corresponding with
the elements in the outer layer). Further, the invention also
relates to the luminescent material obtainable with such method(s)
(see also the other herein described embodiments).
[0065] In yet a further aspect, the invention also provides a
lighting device comprising a light source and a luminescent
material, as defined herein, or obtainable by the method as defined
herein, wherein the light source is configured to illuminate the
luminescent material. The luminescent material, as such, or
embedded in the host material, may be used to convert light source
light into luminescent material light (or converter light). The
luminescent material, such as embedded in the host material, may be
radiationally coupled to the light source. The term "radiationally
coupled" especially means that the light source and the luminescent
material are associated with each other so that at least part of
the radiation emitted by the light source is received by the
luminescent material (and at least partly converted into
luminescence (luminescence material light)).
[0066] The device is especially configured to generate device
light, which at least partly comprises the converter light, but
which may optionally also comprise (remaining) light source light.
For instance, the wavelength converter may be configured to only
partly convert the light source light. In such instance, the device
light may comprise converter light and light source light. However,
in another embodiment the wavelength converter may also be
configured to convert all the light source light.
[0067] Hence, in a specific embodiment, the lighting device is
configured to provide lighting device light comprising both light
source light and converter light, for instance the former being
blue light, and the latter comprising yellow light, or yellow and
red light, or green and red light, or green, yellow and red light,
etc. In yet another specific embodiment, the lighting device is
configured to provide only lighting device light comprising only
converter light. This may for instance happen (especially in
transmissive mode) when light source light irradiating the
wavelength converter only leaves the downstream side of the
wavelength converter as converted light (i.e. all light source
light penetrating into the wavelength converter is absorbed by the
wavelength converter).
[0068] The term "wavelength converter" may also relate to a
plurality of wavelength converters. These can be arranged
downstream of one another, but may also be arranged adjacent to
each other (optionally also even in physical contact as directly
neighboring wavelength converters). The plurality of wavelength
converters may comprise in an embodiment two or more subsets which
have different optical properties. For instance, one or more
subsets may be configured to generate wavelength converter light
with a first spectral light distribution, like green light, and one
or more subsets may be configured to generate wavelength converter
light with a second spectral light distribution, like red light.
More than two or more subsets may be applied. When applying
different subsets having different optical properties, e.g. white
light may be provided and/or the color of the device light (i.e.
the converter light and optional remaining light source light
(remaining downstream of the wavelength converter). Especially when
a plurality of light sources is applied, of which two or more
subsets may be individually controlled, which are radiationally
coupled with the two or more wavelength converter subsets with
different optical properties, the color of the device light may be
tunable. Other options to make white light are also possible (see
also below).
[0069] The terms "upstream" and "downstream" relate to an
arrangement of items or features relative to the propagation of the
light from a light generating means (here the especially the first
light source), wherein relative to a first position within a beam
of light from the light generating means, a second position in the
beam of light closer to the light generating means is "upstream",
and a third position within the beam of light further away from the
light generating means is "downstream".
[0070] The lighting device may be part of or may be applied in e.g.
office lighting systems, household application systems, shop
lighting systems, home lighting systems, accent lighting systems,
spot lighting systems, theater lighting systems, fiber-optics
application systems, projection systems, self-lit display systems,
pixelated display systems, segmented display systems, warning sign
systems, medical lighting application systems, indicator sign
systems, decorative lighting systems, portable systems, automotive
applications, green house lighting systems, horticulture lighting,
or LCD backlighting.
[0071] As indicated above, the lighting unit may be used as
backlighting unit in an LCD display device. Hence, the invention
provides also a LCD display device comprising the lighting unit as
defined herein, configured as backlighting unit. The invention also
provides in a further aspect a liquid crystal display device
comprising a back lighting unit, wherein the back lighting unit
comprises one or more lighting devices as defined herein.
[0072] Preferably, the light source is a light source that during
operation emits (light source light) at least light at a wavelength
selected from the range of 200-490 nm, especially a light source
that during operation emits at least light at wavelength selected
from the range of 400-490 nm, even more especially in the range of
440-490 nm. This light may partially be used by the wavelength
converter nanoparticles (see further also below). Hence, in a
specific embodiment, the light source is configured to generate
blue light.
[0073] In a specific embodiment, the light source comprises a solid
state LED light source (such as a LED or laser diode).
[0074] In yet a further aspect of the invention, a lamp comprises
at least one lighting device according to the invention.
[0075] In yet a further aspect of the invention, a luminaire
comprises at least one lighting device according to the invention
or at least one lamp according to the invention.
[0076] The term "light source" may also relate to a plurality of
light sources, such as 2-20 (solid state) LED light sources. Hence,
the term LED may also refer to a plurality of LEDs.
[0077] The term "substantially" herein, such as in "substantially
all light" or in "substantially consists", will be understood by
the person skilled in the art. The term "substantially" may also
include embodiments with "entirely", "completely", "all", etc.
Hence, in embodiments the adjective substantially may also be
removed. Where applicable, the term "substantially" may also relate
to 90% or higher, such as 95% or higher, especially 99% or higher,
even more especially 99.5% or higher, including 100%. The term
"comprise" includes also embodiments wherein the term "comprises"
means "consists of". The term "and/or" especially relates to one or
more of the items mentioned before and after "and/or". For
instance, a phrase "item 1 and/or item 2" and similar phrases may
relate to one or more of item 1 and item 2. The term "comprising"
may in an embodiment refer to "consisting of" but may in another
embodiment also refer to "containing at least the defined species
and optionally one or more other species".
[0078] Furthermore, the terms first, second, third and the like in
the description and in the claims, are used for distinguishing
between similar elements and not necessarily for describing a
sequential or chronological order. It is to be understood that the
terms so used are interchangeable under appropriate circumstances
and that the embodiments of the invention described herein are
capable of operation in other sequences than described or
illustrated herein.
[0079] The devices herein are amongst others described during
operation. As will be clear to the person skilled in the art, the
invention is not limited to methods of operation or devices in
operation.
[0080] It should be noted that the above-mentioned embodiments
illustrate rather than limit the invention, and that those skilled
in the art will be able to design many alternative embodiments
without departing from the scope of the appended claims. In the
claims, any reference signs placed between parentheses shall not be
construed as limiting the claim. Use of the verb "to comprise" and
its conjugations does not exclude the presence of elements or steps
other than those stated in a claim. The article "a" or "an"
preceding an element does not exclude the presence of a plurality
of such elements. The invention may be implemented by means of
hardware comprising several distinct elements, and by means of a
suitably programmed computer. In the device claim enumerating
several means, several of these means may be embodied by one and
the same item of hardware. The mere fact that certain measures are
recited in mutually different dependent claims does not indicate
that a combination of these measures cannot be used to
advantage.
[0081] The invention further applies to a device comprising one or
more of the characterizing features described in the description
and/or shown in the attached drawings. The invention further
pertains to a method or process comprising one or more of the
characterizing features described in the description and/or shown
in the attached drawings.
[0082] The various aspects discussed in this patent can be combined
in order to provide additional advantages. Furthermore, some of the
features can form the basis for one or more divisional
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0083] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0084] FIG. 1a schematically depicts an embodiment of the reverse
micelle method (copied from H. Ding, Y. Zhang, S. Wang, J. Xu, S.
Xu and G. Li, "Fe.sub.3O.sub.4@SiO.sub.2 Core/Shell Nanoparticles:
The Silica Coating Regulations with a Single Core for Different
Core Sizes and Shell Thicknesses," Chem. Mater., vol. 24, p.
4572-4580, 2012);
[0085] FIG. 1b schematically depicts a quantum dot based
luminescent material;
[0086] FIG. 1c schematically depicts an embodiment of the
luminescent material;
[0087] FIG. 2 evolution of QE (squares) and emission peak
wavelength (crosses) over time (t in seconds) upon exposure of blue
light of a standard silica coated QD sample, without any salt
addition; "PP" indicates peak position (in wavelength (nm));
[0088] FIG. 3 evolution of QE (upper curves) and emission peak
wavelength (lower curves) over time (t in seconds) upon exposure of
blue light of silica coated QDs where either ZnCl.sub.2 (diamond
(1); upper squares (2)) or Na.sub.2S (triangles (3); lower squares
(4)) was added after the silica growth but before the washing
procedure;
[0089] FIG. 4 shows (A-C) TEM images of QDs incorporated in silica
without addition of salts (4A), and where ZnCl.sub.2 was added to
ammonia upfront (4B) a HAADF-TEM image is shown which more clearly
shows the QDs, but also other small particles inside the shell. In
(4C) a zoomed image is shown with red arrows indicating the very
small (.about.2-3 nm) particles (likely ZnCl.sub.2 or ZnO.sub.2) in
the silica shell. (4D) shows a TEM image of silica particles where
Na.sub.2S was added to the ammonia upfront. The image shows that
the QDs are substantially not incorporated in this case;
[0090] FIG. 5 evolution of QE (diamonds (1)) and emission peak
wavelength (squares (2)) over time upon exposure of blue light of
silica coated QDs where ZnCl.sub.2 was added to the ammonia before
silica growth;
[0091] FIG. 6 evolution of QE (triangles (1) and emission peak
wavelength (crosses (2) over time upon exposure of blue light of
silica coated QDs where ZnCl.sub.2 was added to the ammonia before
silica growth, and Na.sub.2S was added after silica growth but
before the washing procedure;
[0092] FIG. 7 evolution of QE (diamonds (1)) and emission peak
wavelength (squares (2)) over prolonged time (t in seconds; 180000
seconds=50 h) upon exposure of blue light of silica coated QDs
where ZnCl.sub.2 was added to the ammonia before silica growth, and
Na.sub.2S was added after silica growth but before the washing
procedure; and
[0093] FIG. 8 schematically depicts an embodiment of a lighting
device.
[0094] The schematic drawings amongst the above figures are not
necessarily on scale.
[0095] FIG. 9a shows a lamp according to the invention and FIG. 9b
shows a luminaire according to the invention.
[0096] FIGS. 10a-10b show stability measurements of Examples 8-9,
respectively, with on the x-axis the time in hours and on the
y-axis the emission intensity in arbitrary units (a.u.).
DETAILED DESCRIPTION OF THE EMBODIMENTS AND EXAMPLES
[0097] FIG. 1a schematically depicts an embodiment of the reverse
micelle method. Reference 31 indicates a QD, reference 32 indicates
a ligand, such as oleic acid or hexadecylamine; reference 33
indicates a surfactant, such as Igepal CO-520, reference 34
indicates a silica formation catalyst such as ammonia, reference 35
indicates a silica precursor, such as TEOS, reference 36 indicates
a hydrolyzed silica precursor, such as hydrolyzed TEOS, reference
37 indicates silica and reference 38 indicates a liquid medium,
especially an organic apolar solvent, such as cyclohexane.
[0098] In FIG. 1a, a schematic overview of most of stages in the
reverse micelle method is shown. First the apolar solvent, such as
cyclo hexane, and the surfactant Igepal.RTM. CO-520 are mixed. Then
the quantum dots, typically with oleic acid as ligands are added,
which results in a ligand exchange with Igepal.RTM. CO-520, which
can be shown by nuclear magnetic resonance. After the addition of
the quantum dots, ammonia (25% in water) is added which is the
catalyst for silica formation (other catalyst can be e.g.
dimethylamine), in which the higher the concentration the faster
the silica growth takes place. At the moment of the addition of
ammonia, micelles will form, in which ammonia is the inner volume
of the micelles and Igepal.RTM. CO-520 is the surfactant. Water
(such as from ammonia) may also necessary for the hydrolysis
reaction of the silica (with ethanol as reaction product), although
it gets released again at the condensation reaction. In the
following step (ammonia addition and TEOS addition can also be
reversed) TEOS is added.
[0099] This silica precursor gets hydrolyzed (partly), due to the
ammonia and a ligand exchange will take place with the Igepal
and/or native ligands on the quantum dot surface. This will also
make the quantum dots water soluble and enables the QDs to be
present within the hydrophilic cores of the micelles. Micelles are
very dynamic which interchange rapidly, however the amount of
quantum dots added should roughly match the typical amount of
silica spheres would be produced in the same synthesis procedure
without QD addition. In this way it is possible to get exactly one
quantum dot per micelle. The fact that the QDs are in the middle of
the silica spheres indicates that the QDs act as seeds for silica
growth. After the ligand exchange by TEOS, the silica will grow
with a speed depending on the ammonia concentration. After several
days all TEOS molecules condensate and the growth stop.
[0100] In another example one can use QDs that have charged ligands
(such as MPA, mercaptopropionic acid) which allows dispersion of
these QDs in water. For these QDs with charged ligands dispersed in
water it was stated that due to the charge of the quantum dot, it
pushed other quantum dots out of the micelle. This electrostatic
repulsion in combination with the correct match of the amount of
quantum dots and silica particles would yield a good precision of
one quantum dot per silica shell.
[0101] In this way, the silica coated QDs can be obtained. After
the last stage, the silica coated QDs can, when desired, be
retrieved from the liquid medium. An option is to add a
precipitator, i.e. a material that induces flocculation and
subsequent precipitation of the QDs. For instance, ethanol can be
used. Thereafter, the QDs can be washed, with a second liquid
medium, such as ethanol or another (organic) (polar) solvent (such
as one or more of acetonitrile, isopropanol, acetone, etc.). Within
ethanol, the QDs can be dispersed and stored. Alternatively, the
QDs can be retrieved from the second liquid medium (see also above)
and can be embedded in a host material (see also below).
[0102] Further, after the silica growth also one or more post
treatments may be applied. Optionally, a further salt treatment
stage may be included. Further, the quantum dots may be subject to
a thermal treatment, which may lead to a more stable QE.
[0103] FIG. 1b schematically depicts a quantum dot based
luminescent material 10. By way of example different types of QDs,
indicated with reference 100, are depicted. The QD at the top left
is a bare QD, without shell. The QD is indicated with C (core). The
QD 10 at the right top is a core-shell particle, with C again
indicating the core, and S indicating the shell. At the bottom,
another example of a core-shell QD is schematically depicted, but a
quantum dot in rod is used as example. Reference 105 indicates the
outer layer, which is in the first example the core material at the
external surface, and which is in the latter two embodiments the
shell material at the external surface of the QD 100.
[0104] FIG. 1c schematically depicts an embodiment of the
luminescent material 10, but now the QDs 100 including the coating
120, especially an oxide coating, such as a silica coating. The
thickness of the coating is indicated with reference d1. The
thickness may especially be in the range of 1-50 nm. Especially,
the coating 120 is available over the entire outer layer 105. Note
however that a silica coating may be somewhat permeable. Note also
that the outer layer 105 of the uncoated nanoparticle (i.e. not yet
coated with the coating of the invention), is (in general) not an
outer layer anymore after the coating process, as then an outer
layer will be the outer layer of the coating 120. However, herein
the term outer layer, especially indicated with reference 105,
refers to the outer layer of the uncoated (core-shell)
nanoparticle.
[0105] Below, some examples are described in more detail.
[0106] A typical QD-silica shell growth is performed by mixing 10
ml cyclohexane and 1.276 ml igepal co-520 in a 20 mL vial under
vigorous stirring. 80 .mu.l TEOS, 1 ml cyclohexane and 12 .mu.l QDs
in heptane (50 mg/ml, CdS core ZnS shell Crystal plex dots
commercially obtained via Crystalplex Inc.) are mixed for around 7
minutes and are afterwards added to the cyclohexane/igepal mixture.
After 15 minutes stirring, 150 .mu.l ammonia (25%) is added which
initiates the reaction. This mixture was stirred vigorously for one
minute to distribute the ammonia evenly over the formed micelles. 1
minute after the ammonia addition, stirring was stopped and the cup
was stored in the dark for typically 2 days. The quantum dots used
are of the core shell type, with the core being Cd(Se,S) and the
shell being ZnS. These quantum dots can be used for both red and
green, with the latter having smaller dimensions. These quantum
dots are provided with alkyl ligands (especially oleic acid) and in
solvent, such as hexane.
[0107] To stop the silica growth, 2.5 ml ethanol was added after
which the QD with silica coating will precipitate and can be
collected by centrifugation. The precipitation was dissolved in 9
ml ethanol and centrifuged again to wash the sample and remove
unwanted reagents. This was repeated for two more times. Finally
the sample was stored in ethanol and sonication was used disperse
the QD with silica coating until a clear solution was obtained.
[0108] Silica growth takes place in the (basic) aqueous phase
within the reverse micelle, and QDs can act as seeds for shell
growth. Therefore, at the appropriate ratio of QDs and micelles it
is possible to incorporate single QDs in the middle of individual
silica particles of roughly 20-25 nm in diameter, see FIG. 4a. It
appears that the QDs with a hydrophobic oleic acid capping, which
disperse well in apolar solvents like toluene or heptanes, act as
seeds in the aqueous phase of the reverse micelle. It seems that
hydrolyzed TEOS molecules (i.e. Si--O.sup.- groups) can replace the
oleic acid molecule (the native ligand). The TEOS-capped QDs have a
much better affinity for the polar aqueous phase, and can therefore
act as seed of silica growth.
[0109] In the examples below, it is shown that the QE of QDs
typically drops from 80% to 20-30% upon silica shell growth using
the standard recipe. However, the QE can be improved to 50% if
salts such as ZnCl.sub.2 or Na.sub.2S are added during or after
silica shell growth. The light exposure effect (sometimes also
called "photobrightening effect) and peak wavelengths are also
affected depending on the exact addition procedure and material.
The results clearly show that the added salts have an effect on the
QD performance. Eventually, a QE of above 70% could be measured (in
air) using the modified silica shell growth procedure and applying
a light exposure step.
Example 1: QE of Baseline QD-Silica Samples
[0110] As a baseline, first a silica shell was grown around QDs
using the standard recipe as described above. Aliquots were taken
during silica shell growth, and the QE and peak wavelength were
measured. Table 1 gives an overview of the QE as function of time.
It shows that the QE drops from 80% of the native dots, towards
20-30% after silica shell growth. The drop predominantly takes
place within the first 5 minutes of silica shell growth, which
confirms that this is due to a QD surface/ligand effect. The
resulting silica coated QDs using this recipe are shown in FIG.
1.
TABLE-US-00001 TABLE 1 evolution of the quantum efficiency and peak
position during silica shell growth. Peak position Time after
quenching [min] Quantum efficiency [%] [nm] Original dots 2.5 mg/ml
80.2 609.73 5 39.7 608.73 150 26.2 607.06 600 29.8 605.10 1147 29.7
604.47 2880 31.9 604.18 10080 19.2 604.09
[0111] FIG. 2 shows how the QE of liquid samples of a typical
"baseline" silica coated QD sample changes upon exposure of blue
light. Moderate light exposure is observed, with a final QE up to
35%. The light exposure is typically accompanied by a red-shift in
emission wavelength.
[0112] The standard light exposure herein is executed by exposure
to (blue) light at medium flux (.about.1 W/cm.sup.2). In this way,
the light exposure effect or photobrightening effect may be
evaluated as sometimes the QE increases (photobrightening), or
sometimes also decreases, under light exposure.
Example 2: Addition of ZnCl.sub.2 or Na.sub.2S after Silica Growth
but Before the Washing Procedure
[0113] To investigate the influence of salts on the QE of QDs,
ZnCl.sub.2 or Na.sub.2S were added after the silica shell growth
was completed (after 20 hours of shell growth in the dark). For
these experiments a silica shell growth experiment was performed as
described in Example 1, but after 20 hours of shell growth the
mixture was divided over several vials for multiple addition
experiments on the same baseline QD-silica mixture. For both salts,
100 .mu.l of a certain concentration salt in water was added to the
reverse micelle mixture under mild stirring. This was done before
the washing procedure was performed, in other words the micelles
were still intact. After 1 hour of stirring with the added salt,
the mixture was washed using the standard procedure, and the
particles were redispersed in Ethanol after which the QE was
determined. The QE's are listed in Table 2, together with the peak
wavelength of the QD emission spectrum. The reference samples have
a QE of 33% and 35%, which is slightly higher than the result
described above but within the same range. When ZnCl.sub.2 is
added, a gradual increase in QE is observed with increasing salt
concentration (up to 47%), together with a red-shift in the
emission peak wavelength (up to 2 nm). Similarly, an increase in QE
to 49% is observed upon addition of a 100 mM Na.sub.2S solution,
however 400 mM shows an increase to only 39%. Albeit a small
change, the emission peak wavelength shifts to the blue in this
case by approximately 0.5-1 nm.
[0114] The results above show that addition of a salt has an impact
on the final QE and peak wavelength of the QDs. It suggests that
the ions have the chance to diffuse through the (porous) silica
particle towards the QD surface.
TABLE-US-00002 TABLE 2 QE data of silica coated QDs with and
without addition of 100 .mu.l of ZnCl.sub.2 or Na.sub.2S salt at
various concentrations. Na.sub.2S ZnCl.sub.2 Quantum Concentration
Quantum PL peak efficiency PL peak [mM] efficiency [%] [nm] [%]
[nm] 0 (reference 1) 35 604.8 35 604.8 0 (reference 2) 33 604.5 33
604.5 25 36 604.8 n.m. n.m. 100 39 605.3 49 604.5 400 47 606.6 38
604.0 n.m. = not measured
[0115] The QE of the silica coated QDs where ZnCl.sub.2 or
Na.sub.2S was added after silica growth but before the washing
procedure was also followed over time upon illumination with blue
light. In both cases 100 .mu.l of 400 mM salt concentration was
added. The QE of both samples start at 47% and 40%, in line with
the results listed in table 2. The sample treated with ZnCl.sub.2
shows light exposure up to 60% QE, accompanied by a red-shift of
.about.1 nm. The sample treated with Na.sub.2S first shows a
dramatic drop in QE to .about.15%, after which it increases to
.about.40%, again accompanied by a red-shift of in this case
.about.2 nm. From these results it appears that a ZnCl.sub.2
treatment after silica growth is a good route.
Example 3: Addition of ZnCl.sub.2 and Na.sub.2S after Silica Growth
but Before the Washing Procedure
[0116] Silica coated QDs from reference 1 were also used to add
both ZnCl.sub.2 and Na.sub.2S, in different sequence, see Table 3.
The results of the reference and ZnCl.sub.2-only and Na.sub.2S-only
experiments are also given for comparison. When first ZnCl.sub.2 is
added and then Na.sub.2S, a QE of 52% is measured, with a peak
wavelength of 604.5 nm. When the salts are added in the reverse
sequence, a QE of 41% is measured, with a peak wavelength of 605.2
nm. These results confirm the effect of salt addition on QE and
peak wavelength, and show that the sequence in which the salts are
added do impact the final optical properties of the QDs. The
results show that in case of adding both salts, it is most
favorable to add Na.sub.2S as last.
TABLE-US-00003 TABLE 3 overview of QE upon addition of ZnCl.sub.2
and Na.sub.2S after silica growth but before the washing procedure
Addition sequence QE [%] Peak position [nm] Reference 1 35 604.8
ZnCl.sub.2 100 mM 39 605.3 Na.sub.2S 100 mM 49 604.6 ZnCl.sub.2
=> Na.sub.2S 100 mM 52 604.5 Na.sub.2S => ZnCl.sub.2 100 mM
41 605.2
Example 4: Addition of ZnCl.sub.2 or Na.sub.2S During Silica Shell
Growth
[0117] In another embodiment the ZnCl.sub.2 or Na.sub.2S were added
during the silica growth, where the 100 mM solutions of theses
salts were mixed with the ammonia upfront. In this experiment 13 mg
(0.93*10.sup.-4 mol) of ZnCl.sub.2 per 150 .mu.L of 35% ammonia was
used. Similarly, 50.76 mg Na.sub.2S was added to 150 .mu.L ammonia
35%. Na.sub.2S is a strong base, so also lower ammonia
concentration could be useful. TEM images of the results are
displayed in FIG. 4 below, in which (A-C) represents the ZnCl.sub.2
addition and (D) the Na.sub.2S addition.
[0118] The TEM results in FIG. 4 show that ZnCl.sub.2 addition to
the ammonia before addition to the silica synthesis results in the
formation of small nanoparticles in the shell of the silica
particles. These particles could be ZnCl.sub.2 and/or ZnO
particles, but there is no conclusive evidence for this. The QDs
are however still well incorporated in the silica particles. In
case Na.sub.2S is added to the ammonia before addition to the
silica synthesis, it appears that the QDs are not incorporated into
the silica particles, but attached to the outside of the silica
particles. It is suggested that this is due to a too high pH during
silica growth, because Na.sub.2S is a strong base by itself.
[0119] The QE of the sample where ZnCl.sub.2 was added to the
ammonia had a QE of 51% (peak at 606.9 nm) and a remake had a QE of
54% (peak at 606.4 nm). FIG. 5 below shows the that the QE of this
sample did not show light exposure (but rather a drop) upon
exposure to blue light, accompanied by a red-shift of .about.1.5
nm. This is somewhat remarkable because the sample where the
ZnCl.sub.2 was added to the reaction mixture after silica growth
(example 2) did show light exposure effects.
[0120] Hence, the invention also provides a luminescent material
comprising particles and/or agglomerates of particles, wherein the
particles especially have dimensions in the range of 20-500 nm,
such as especially 20-350 nm, wherein substantially each particle
comprises a (single) quantum dot surrounded by a silica coating. As
indicated above, the quantum dot can be of the core-shell type. The
silica comprising shell comprises also one or more elements that
are shared with the outer layer of the quantum dot. Especially,
about at least 70 wt. % of the luminescent material may comprise
such particles and/or agglomerates thereof. Hence, the individual
particles may not necessarily be interconnected. The individual
particles may agglomerate, but may not form a combination of
quantum dots sharing a single silica coating. Part of the
luminescent material, such as 30 wt. % or less may optionally be
based on particles sharing two or more quantum dots.
Example 5: Addition of ZnCl.sub.2 During Silica Shell Growth, and
Na.sub.2S after Silica Growth but Before the Washing Procedure
[0121] In yet another embodiment, QDs were coated with a silica
shell where ZnCl.sub.2 was added together with the ammonia, and
Na.sub.2S was added after silica growth but before the washing
procedure. In this experiment, 13 mg of ZnCl.sub.2 was dissolved
into the ammonia (35%) solution, of which 150 ul was added at the
start of the silica growth. After the silica synthesis was
completed (22 hours in the dark without stirring), 100 ul of 400 mM
Na.sub.2S in water was added to the reaction mixture and stirred
gently for 1 hour. Next, the reaction mixture was precipitated with
ethanol and washed 3 times in ethanol. The QE of the resulting
sample was 54% at a peak wavelength of 608 nm. FIG. 6 below shows
that this sample does show light exposure up to 58% within 1400 s,
together with a red-shift in emission peak wavelength of .about.0.7
nm. Note that this peak wavelength is clearly different from the
sample described in example 4, where ZnCl.sub.2 addition without
Na.sub.2S post treatment resulted in an initial peak at 606.5
nm.
[0122] When the QE of the same sample was measured again, a QE of
66% was measured with a peak wavelength of 608.5 nm. When this
sample was exposed to blue light for prolonged times (up to 2 days)
light exposure was observed up to a QE of 76%, while the peak
wavelength hardly shifts anymore; see FIG. 7.
Example 6: Application in Different Films of Red QD
[0123] A Comparison between a QD dispersion in toluene and in
silicone films (wavelength converter element). Samples A-E were
treated with 40 .mu.L ZnCl.sub.2 (0.4 M) and/or 100 .mu.L Na.sub.2S
(0.4 M). A sample F was treated with 40 .mu.L saturated ZnCl.sub.2
and 100 .mu.L Na.sub.2S (0.4 M)
Both samples treated with light exposure (LE) had an increase of
the QE towards 65-70%. The results are indicated in table 4:
TABLE-US-00004 TABLE 4 QD in toluene and in film for red QDs
Dispersion Film Dispersion Film t = 0 t = 0 t = 1 t = 7 t = 0 t = 0
t = 1 t = 7 days days days days days days days days QE QE QE QE
.lamda..sub.max .lamda..sub.max .lamda..sub.max .lamda..sub.max
Sample Treatment (%) (%) (%) (%) (nm) (nm) (nm) (nm) A SiO.sub.2
28.3 17.6 18.5 18.0 603.02 605.56 605.41 604.91 B SiO.sub.2 +
ZnCl.sub.2 44.8 24.4 26.8 22.5 605.34 607.00 606.92 606.67 C
SiO.sub.2 + Na.sub.2S 36.4 30.1 31.4 32.1 602.28 603.81 603.54
602.92 D SiO.sub.2 + ZnCl.sub.2 + 48.4 28.6 30.6 30.8 605.02 605.77
605.55 605.26 Na.sub.2S E SiO.sub.2 + ZnCl.sub.2 + 42.3 43.1 45.8
47.2 604.55 605.25 605.18 604.87 Na.sub.2S + PB F SiO.sub.2 +
ZnCl.sub.2 + 56.4 24.7 27.7 28.5 609.58 609.15 609.13 608.47
Na.sub.2S + PB
[0124] It appears that the samples treated with Na.sub.2S show
constant QE in films. Further, ZnCl.sub.2 induces a redshift and
Na.sub.2S induces a blueshift. Further, light exposure has a
positive effect on the QE in the films. Sample F has the optimal
treatment w.r.t. QE for dispersions, and sample E has the optimal
treatment w.r.t. QE for films.
Example 6: Application in Different Films of Green QD
[0125] The same experiment was applied, but now with green QDs
(NC536A), indicated with samples G-L, see table 5:
TABLE-US-00005 TABLE 5 QD in toluene and in film for green QDs
Dispersion Film Dispersion Film t = 0 t = 0 t = 1 t = 7 t = 0 t = 0
t = 1 t = 7 days days days days days days days days QE QE QE QE
.lamda..sub.max .lamda..sub.max .lamda..sub.max .lamda..sub.max
Sample Treatment (%) (%) (%) (%) (nm) (nm) (nm) (nm) G SiO.sub.2
17.8 10.3 10.6 10.0 538.18 541.00 540.89 540.32 H SiO.sub.2 +
ZnCl.sub.2 30.5 11.8 13.3 14.0 539.50 543.12 542.94 542.73 I
SiO.sub.2 + Na.sub.2S 21.2 20.8 22.9 24.5 537.72 539.91 539.75
539.49 J SiO.sub.2 + ZnCl.sub.2 + 45.5 16.4 17.1 18.3 537.91 538.87
538.80 538.61 Na.sub.2S K SiO.sub.2 + ZnCl.sub.2 + 42.2 26.2 26.7
28.2 538.17 539.18 539.06 538.81 Na.sub.2S + PB L SiO.sub.2 +
ZnCl.sub.2 + 45.9 20.2 21.8 24.3 545.13 544.60 544.51 544.58
Na.sub.2S + PB
[0126] Also these samples show constant QE in films. Further, again
it was perceived that ZnCl.sub.2 induces a redshift and that
Na.sub.2S induces a blueshift. Also here it was observed that light
exposure has a positive effect on the QE in the films. Sample K has
the optimal treatment w.r.t. QE for dispersions and sample L has
the optimal treatment w.r.t. QE for films.
[0127] The results above demonstrate that silica coating of QDs
results in a dramatic drop of QE from 80% to 20-30%. Addition of
ZnCl.sub.2 and/or Na.sub.2S salts during or after silica growth can
improve the QE up to 60% or higher. Light exposure can further
increase the QE to above 75%. Without salt addition, light exposure
is limited up to 35%. It is suggested that during silica growth, Zn
and/or S ions can be removed from the QD surface, resulting in
surface defect states, reducing QE. By adding Zinc and/or sulphide
salts, (and/)or other relevant salts, during or after silica shell
growth, these traps states may be recovered. The changes in QE but
also shifts in peak wavelength support the idea that the Zn and/or
Sulphide ions, (and/)or other relevant ions, can actually migrate
into the silica sphere towards the QDs, and thereby affect the
optical properties. It is expected the quantum efficiencies can
even be further improved by finetuning temperatures, concentrations
and salt combinations and or salt addition schemes.
[0128] Below in table 6, the chemical used are defined:
TABLE-US-00006 TABLE 6 List of chemicals used Chemical Abbreviation
Manufacturer Purity M (g/mol) Cyclohexane C.sub.6H.sub.12 Merck
.gtoreq.99.5% 84.16 Tetraethyl Si(OC.sub.2H.sub.5).sub.4 Sigma
Aldrich .gtoreq.99.0% 208.33 orthosilicate Quantum dots QDs
Crystalplex NC- -- 50 mg/ml 610-A_5 Quantum dots QDs Crystalplex
NC- 50 mg/ml 536A Zinc Chloride ZnCl.sub.2 Sigma Aldrich
.gtoreq.97% 36,315 Igepal .RTM. CO-520
(C.sub.2H.sub.4O).sub.n.cndot.C.sub.15H.sub.24O, Sigma Aldrich --
441 n~5 Ammonia 35% in NH.sub.4OH Fisher Electronic (MOS) 35.04
water grade Sodium sulfide Na.sub.2S.sub.n.cndot.9H.sub.2O Sigma
Aldrich .gtoreq.99.99% 240.18 nonahydrate Formamide CH.sub.3NO
Sigma Aldrich .gtoreq.99.5% 45.04 Ethanol C.sub.2H.sub.6O Bisolv
Chimie semiconductor 46.0 (dehydrated) grade MOS PURANAL
[0129] FIG. 8 depicts a lighting device 150 with a light source
160, configured to generate light source light 161. This light
source light 161 is at least partly received with the luminescent
material 10, for instance in the form of a layer or body 1000, or
comprised by such layer or body 1000. This layer or body may also
be indicated as wavelength converter element (see also FIG. 2a).
The luminescent material 10 is optically coupled with the light
source 160. The luminescent material absorbs at least part of the
light source light 161 and converts this light source light 161
into luminescent material light. The light provided by the lighting
device 150 is indicated with reference 151. This lighting device
light 151 may at least include the light generated by the
luminescent material 10 upon said excitation with the light source
light 161, but may optionally also include said light source light
161. Reference d2 indicates the distance of the downstream arranged
luminescent material, here embedded in the wavelength converter
comprising. The distance may be non-zero, indicating e.g. a remote
configuration. The distance may optionally also be zero.
[0130] FIG. 9a shows a lamp 900 that comprises a lighting device
according to FIG. 8. In an alternative embodiment the lamp 900 may
comprise multiple lighting devices according to the invention.
[0131] FIG. 9b shows a luminaire 950 that comprises a lighting
device according to FIG. 8. In an alternative embodiment the
luminaire 950 may comprise multiple lighting devices according to
the invention, or one or more lamps according to FIG. 9a.
Example 7: Addition of Cd(NO.sub.3).sub.2 During Silica Growth
[0132] In yet another example, QDs were coated with a silica shell
where of Cd(NO.sub.3).sub.2 was added together with the ammonia. In
this experiment, 40 .mu.l of Cd(NO.sub.3).sub.2 in water added with
120 .mu.l ammonia (35%), at the start of the silica growth. After
the silica synthesis was completed (22 hours in the dark without
stirring). Next, the reaction mixture was precipitated with ethanol
and washed 3 times in ethanol. After washing the reaction mixture
was dispersed in 400 .mu.l toluene. The QE of the resulting sample
was 71% at a peak wavelength of 609 nm. Note that this peak
wavelength is clearly different from the sample described in
example 4, where ZnCl.sub.2 addition resulted in an initial peak at
606.5 nm.
Example 8: Addition of Cd(NO.sub.3).sub.2 During Silica Growth and
Na.sub.2S Addition after Silica Growth but Before the Washing
Procedure
[0133] In yet another example, QDs were coated with a silica shell
where Cd(NO.sub.3).sub.2 was added together with the ammonia, and
Na.sub.2S was added after silica growth but before the washing
procedure. In this experiment, 0.5 M of Cd(NO.sub.3).sub.2 was
added with 120 .mu.l of ammonia (35%) solution. After the silica
synthesis was completed (22 hours in the dark without stirring),
100 .mu.l of 400 mM Na.sub.2S in water was added to the reaction
mixture and stirred gently for 30 minutes followed by the addition
of 100 .mu.l of 0.8M NaOH for 30 minutes. Next, the reaction
mixture was precipitated with ethanol and washed 3 times in ethanol
and redispersed in toluene. The QE of the resulting sample was 82%
at a peak wavelength of 606 nm. When the quantum dots were
incorporated in a silicone film, the QE dropped to 61%. In an
accelerated degradation test the samples was measured in an
accelerated testing protocol on a LED with drive current of 200 mA
and a board temperature of 90.degree. C. After the thermal quench,
related to the high operating temperature the emitted intensity
dropped <15% over 60 hours as shown in FIG. 10a.
Example 9: Addition of ZnCl.sub.2 and Na.sub.2S Addition after
Silica Growth but Before the Washing Procedure
[0134] In yet another example, QDs were coated with a silica shell
where ZnCl.sub.2 was added together with the ammonia, and Na.sub.2S
was added after silica growth but before the washing procedure
followed by the addition of base. In this experiment, 50 .mu.l 0.1M
ZnCl.sub.2 was added with 150 .mu.l of the ammonia (35%) solution
at the start of the silica growth. After the silica synthesis was
completed (22 hours in the dark without stirring), 100 .mu.l of 400
mM Na.sub.2S in water was added to the reaction mixture and stirred
gently for 30 minutes followed by addition of 50 .mu.l 0.8 M NaOH
and another 30 minutes stirring. Next, the reaction mixture was
precipitated with ethanol and washed 3 times in ethanol and
redispersed in toluene. The QE of the resulting sample was 58% at a
peak wavelength of 605 nm. The emission as a function of time is
followed at 200 mA and 90.degree. C., as shown in FIG. 10b. After
initial photo brightening and thermal quenching effects, the red
emission signal is constant within 10% for at least 150 hours.
[0135] Note that the embodiment of Example 9 has a higher stability
than the embodiment of Example 8, and also a smaller intensity drop
is perceived.
Example 10: Analysis of Amounts of Ions after ZnCl.sub.2,
Na.sub.2S, NaOH Treatment of QDs from Example 1 and Example 9
[0136] ICP-MS analysis of a sample prepared according to example 1
was carried out and compared to that of sample prepared according
to example 9.
[0137] In order to release the elements of interest out of the
matrix, the sample is destructed using a microwave digestion
procedure. After dissolution, the sample is diluted to a known
volume and a semi-quantitative measurement was performed in order
to determine the present elements by means of Inductively Coupled
Plasma-Optical Emission Spectrometry (ICP-OES). Subsequently the
determined elements are determined more precisely by means of
ICP-OES.
[0138] During ICP-OES analysis, the sample solutions are fed
through a nebulizer. This produces an aerosol which is led into an
argon plasma. In this plasma the solution is evaporated, atomized
and excited, which produces element specific emission. The
intensity and wavelength of the emission is used to determine the
amounts of Cd, K, Na, S, Si and Zn present. Calibration is
performed by comparison with the intensities produced by a matrix
matched blank and at least four matrix matched calibration standard
solutions (obtained by dilution of certified reference
solutions).
[0139] Each blank, calibration standard and sample solution
contains corresponding amounts of internal standards to correct for
system variations during measurement. Each ICP-OES measurement
consists of multiple replicates and each is measured using several
wavelengths. For quality control blanks and spike recovery
experiments are taken along.
[0140] The results of the quantitative ICP-OES analyses are
presented in table 7. The concentrations are expressed in ratios of
weight percentages (wt %) relative to the amount of silicon. Each
sample is analyzed in triplicate. The amounts of Na, S and Zn, are
significantly higher for Example 9 compared to Example 1. The Cd
and K content are the same within experimental error while the
amounts of Na, S and Zn, reach values up to 10-20% of the amount if
Si
TABLE-US-00007 TABLE 7 weight of Cd, K, Na, S and Zn relative to
the weight of silicon. Element Example 1 Example 9 Cd 4.3 4.5 Na
0.3 17.9 S 1.8 11.2 Si 100 100 Zn 2.6 15.7
[0141] The invention enables efficient (QD-converted) LED light
sources at high flux densities, for example LED lamps, spot lights,
outdoor lighting, automotive lighting and/or laser
applications.
* * * * *